The present invention relates to a cleaning device for an image forming apparatus of the type developing an electrostatic latent image formed on an image carrier with toner, transferring the resulting toner image onto a transfer material, and cleaning the image carrier to remove remaining toner. More particularly, the present invention is concerned with a magnet brush type cleaning device which includes an exclusive member for retaining carrier thereon by a magnetic force.
There has been known an image forming apparatus which develops an electrostatic latent image provided on a photoconductive element or similar image carrier with toner, transfers the resulting toner image onto a transfer material, and repeats such a sequence of steps thereafter. A prerequisite with such an apparatus is that toner remaining on the image carrier after the transfer of toner image be removed to prepare the image carrier for another image forming cycle. A cleaning device of the type using a magnet brush has been proposed for removing the remaining toner, as disclosed in Japanese Patent Publication No. 48-37382 and Japanese Laid-Open Patent Publication (Kokai) Nos. 56-51768 and 50-75044 by way of example. This type of cleaning device includes a carrier retaining member in the form of a cleaning sleeve and a plurality of magnets which are accommodated in the carrier retaining member. A bias voltage opposite in polarity to the charge of remaining toner is applied to the carrier retaining member to charge carrier which is deposited on the retaining member, whereby the remaining toner whose polarity is opposite to that of the carrier is adhered to the carrier by an electric force and thereby removed from the image carrier. Such a magnet brush type cleaning device is advantageous over a fur brush type cleaning device, blade type cleaning device and others in that it does not noticeably damage the image carrier and preserves the cleaning effect over a long period of time.
To enhance the cleaning effect of a magnetic brush type cleaning device, it is preferable that the toner remaining on the image carrier be attracted by an intense electric force toward the carrier. A possible approach for achieving such an object is reducing the volume resistivity of the carrier which is retained on the carrier retaining member or cleaning sleeve, more specifically the volume resistivity of at least the surface portions of the individual carrier particles, to smaller than 10.sup.10 .OMEGA..cm. Then, the bias voltage applied to the cleaning sleeve will increase the effective bias at the tip of the carrier and thereby intensify the electric field between the carrier and the image carrier. In this condition, the toner on the image carrier and existing in such an electric field is attracted by an intense electric force toward the carrier. This kind of approach is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 50-75044. However, when carrier having such low resistance is used and deposited with a larger amount of charge, there is a fear that the charge on the carrier is injected into the toner which is adhered to the carrier, inverting the polarity of charge of the toner. This causes the carrier to repulse the toner away from the cleaning sleeve toward the image carrier and thereby lowers the cleaning ability.
In light of the above, it has been customary to use carrier whose volume resistivity is greater than 10.sup.10 .OMEGA..cm so as to increase the amount of frictional charge developing between the carrier and the toner. The charge deposited on such carrier serves to attract the remaining toner on the image carrier toward the carrier. With this kind of scheme, however, it is impossible to achieve a sufficient cleaning effect. Moreover, in order that the amount of frictional charge between the carrier and the toner may be increased, the rotation speed of the cleaning sleeve and/or that of the magnets disposed in the cleaning sleeve has to be increased to intensity the agitating force which acts on the carrier and toner. Increasing the rotation speed as mentioned subjects the carrier to excessive external forces and thereby aggravates the deterioration of the carrier, resulting in poor cleaning ability.